Leishmania braziliensis, growing axenically at 26 C and transferred to 34 C, changes within 3 hr from the long slender motile promastigote form to an ellipsoidal form with a nonmotile flagellum. This transformation is reversible for heat treatments of up to 12 hr. In this study we show by light microscopic measurements that the cells decrease in length and increase in diameter at constant volume. Quantitative morphometry of electron micrographs further demonstrates that: the distance between nucleus and kinetoplast decreases; the kinetoplast enlarges slightly; the distance between adjacent subpellicular microtubules decreases; and that after 3 hr of heat treatment there is no change in mitochondrial morphology, but after 6 hr of heat treatment the mitochondria lose their cristae and no longer possess a clearly defined double membrane. These observations are compared with the morphological changes that occur normally in the gut of a sandfly and in the in vivo transformation occurring during infection of the mammalian host and of macrophage cultures.